Ink-jet printing has become an established printing technique that generally involves the controlled delivery of ink drops from an ink reservoir to a printing surface.
One type of ink-jet printing, known as drop-on-demand printing, employs a pen that has a print head that is responsive to control signals for ejecting drops of ink from the ink reservoir. Drop-on-demand ink-jet pens typically use one of two mechanisms for ejecting drops: thermal bubble or piezoelectric pressure wave.
The print head of a thermal bubble type pen includes a thin-film resistor that is heated to cause sudden vaporization of a small portion of the ink. The rapid expansion of the ink vapor forces a small amount of ink through a print head orifice.
Piezoelectric pressure wave pens use a piezoelectric element that is responsive to a control signal for abruptly compressing a volume of ink in the print head to thereby produce a pressure wave that forces the ink drops through the orifice.
Although conventional drop-on-demand print heads are effective for ejecting or "pumping" ink drops from a pen reservoir, they do not include any mechanism for preventing ink from permeating through the print head when the print head is inactive. Accordingly, drop-on-demand techniques require that the fluid in the ink reservoir must be stored in a manner that provides a slight underpressure within the reservoir to prevent ink leakage from the pen whenever the print head is inactive. As used herein, the term underpressure means that the fluid pressure within the reservoir is less than the pressure of the ambient air surrounding the reservoir. A rise in underpressure means the fluid pressure in the reservoir becomes more negative relative to ambient air.
The underpressure in the reservoir must be great enough for preventing ink leakage through the print head. The underpressure, however, must not be so great that the pumping action of the print head cannot overcome the underpressure to eject ink drops.
The underpressure of an ink-jet pen reservoir changes as the print head is activated to eject drops. Specifically, the ejection of ink from the reservoir increases the reservoir underpressure. Without regulation of such underpressure increase, the ink-jet pen will eventually fail because the print head will be unable to overcome the increased underpressure to eject the ink drops.